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              <h1>JVM垃圾回收机制 - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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  {
    "question": "根据文档，引用计数法（Reference Counting）在判断对象是否为垃圾时存在一个主要缺陷，这个缺陷是什么？",
    "options": [
      "算法执行效率过低，导致系统长时间停顿。",
      "无法处理对象之间相互循环引用的问题。",
      "会占用大量额外的内存来存储引用计数。",
      "只能在单线程环境下工作，不适用于现代多核CPU。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "引用计数法的主要问题是，当两个或多个对象相互引用时，即使它们已经没有被任何外部对象引用（即从GC Roots不可达），它们的引用计数仍然不为零。这会导致垃圾回收器无法回收这些实际上已经无用的对象，从而造成内存泄漏。",
    "source": "文档中「回收算法【定位那个对象是垃圾】」-> 「引用计数法」部分",
    "hint": "文档中提供了一个`MyObject`的例子，其中`object1`和`object2`互相持有对方的引用，即使最后它们都被设置为`null`，它们的引用计数也无法归零。"
  },
  {
    "question": "在可达性分析法中，JVM从一系列“GC Roots”出发来追踪存活对象。以下哪一项不属于文档中明确列出的GC Roots？",
    "options": [
      "虚拟机栈（栈帧中的本地变量表）中引用的对象。",
      "方法区中静态属性引用的对象。",
      "堆内存中被老年代（Old Generation）对象引用的新生代（Young Generation）对象。",
      "本地方法栈中（Native方法）引用的对象。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "文档中明确列举了四种GC Roots：虚拟机栈中引用的对象、方法区中静态属性引用的对象、方法区中常量引用的对象以及本地方法栈中引用的对象。虽然在某些跨代回收的场景下，老年代对象可以作为新生代回收的“根”，但这并不是文档中定义的通用GC Roots基本类型。",
    "source": "文档中「回收算法【定位那个对象是垃圾】」-> 「可达性分析法」部分",
    "hint": "回顾文档中标题为“什么是 GC Roots 呢？”下方的有序列表，检查哪个选项不在其中。"
  },
  {
    "question": "“标记-清除”（Mark-Sweep）算法是垃圾回收的基础算法之一，但它有一个显著的缺点。这个缺点是什么？",
    "options": [
      "它需要将内存分为大小相等的两块，导致内存利用率减半。",
      "回收后会产生大量不连续的内存碎片。",
      "标记和清除过程都需要暂停所有用户线程（Stop-The-World）。",
      "算法实现非常复杂，且效率低下。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "标记-清除算法在清除掉被标记为垃圾的对象后，并不会移动存活的对象来整理内存空间。这导致剩余的内存是不连续的碎片。当程序需要分配一个较大的连续内存空间时，即使总的可用内存足够，也可能因为找不到足够大的连续块而失败。",
    "source": "文档中「回收方法【回收垃圾】」-> 「标记-清除算法」部分",
    "hint": "文档中提到该算法的缺点是“……后期运行过程中需要分配较大对象时无法找到足够的连续内存……”。"
  },
  {
    "question": "分代收集算法是现代JVM普遍采用的策略。针对“新生代”（Young Generation）中对象“朝生夕死”的特点，通常会选用哪种回收算法？",
    "options": [
      "标记-整理算法（Mark-Compact）",
      "复制算法（Copying）",
      "标记-清除算法（Mark-Sweep）",
      "引用计数算法（Reference Counting）"
    ],
    "correctAnswerIndex": 1,
    "explanation": "由于新生代中的绝大多数对象生命周期都很短，每次GC后只有少量对象存活。使用复制算法只需将这些少量存活的对象复制到另一块内存区域即可，然后一次性清空原来的区域。这种方式效率高，且不会产生内存碎片，非常适合新生代的特点。",
    "source": "文档中「回收方法【回收垃圾】」-> 「分代收集算法」部分",
    "hint": "文档总结中提到：“新生代中：每次垃圾收集时都有大批对象死去，只有少量存活，那就选用……”。"
  },
  {
    "question": "Java对引用的概念进行了扩充，分为四种强度。哪种引用类型的对象生命周期最顽强，只要引用存在，垃圾回收器就永远不会回收它？",
    "options": [
      "软引用（Soft Reference）",
      "弱引用（Weak Reference）",
      "强引用（Strong Reference）",
      "虚引用（Phantom Reference）"
    ],
    "correctAnswerIndex": 2,
    "explanation": "强引用是我们编程中最常见的引用类型，如 `Object obj = new Object()`。只要一个对象有强引用指向它，垃圾回收器就绝对不会回收该对象，即使系统内存空间不足，宁可抛出OutOfMemoryError异常也不会回收。",
    "source": "文档中「四种引用状态」-> 「1、强引用」部分",
    "hint": "文档中描述这种引用时使用了“永远不会回收掉被引用的对象”这样的表述。"
  },
  {
    "question": "根据文档描述，哪种引用类型关联的对象会在下一次垃圾回收发生时，无论当前内存是否充足，都会被回收？",
    "options": [
      "软引用（Soft Reference）",
      "强引用（Strong Reference）",
      "虚引用（Phantom Reference）",
      "弱引用（Weak Reference）"
    ],
    "correctAnswerIndex": 3,
    "explanation": "弱引用（WeakReference）的强度比软引用更弱。被弱引用关联的对象只能存活到下一次垃圾回收发生之前。当垃圾回收器开始工作时，一旦发现只具有弱引用的对象，不管当前内存空间是否足够，都会回收其内存。",
    "source": "文档中「四种引用状态」-> 「3、弱引用」部分",
    "hint": "文档中明确指出这种引用的特点是“……无论当前内存是否足够，都会回收掉只被弱引用关联的对象。”"
  },
  {
    "question": "文档中提到了多种JVM垃圾回收器，其中哪一个回收器的主要目标是“获取最短回收停顿时间”，因此特别适合需要快速响应的互联网应用？",
    "options": [
      "Parallel Scavenge",
      "Serial Old",
      "CMS (Concurrent Mark Sweep)",
      "G1 (Garbage First)"
    ],
    "correctAnswerIndex": 2,
    "explanation": "CMS（Concurrent Mark Sweep）收集器是一款针对老年代的、以获取最短回收停顿时间为目标的收集器。它允许垃圾收集线程与用户线程在大部分时间内并发执行，从而大大缩短了因GC引起的“Stop-The-World”停顿时间，适用于对响应时间有较高要求的应用。",
    "source": "文档中「JVM垃圾回收器」部分",
    "hint": "在垃圾回收器的列表中，寻找那个被描述为“以获取最短回收停顿时间为目标的收集器”的选项。"
  },
  {
    "question": "文档指出，G1（Garbage First）收集器与之前的收集器相比有一个重要特点，这个特点是什么？",
    "options": [
      "它是第一个单线程的收集器，追求极致的简单高效。",
      "它只负责新生代的回收，需要搭配老年代收集器使用。",
      "它的回收范围是整个Java堆（包括新生代和老年代）。",
      "它完全基于“标记-清除”算法，回收速度最快。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "与之前的新生代和老年代收集器分离的设计不同，G1收集器是一款面向整个Java堆的垃圾回收器。它将堆划分为多个大小相等的独立区域（Region），并跟踪这些区域里垃圾的价值大小，在后台维护一个优先列表，优先回收价值最大的区域，从而实现了对回收时间的精确控制。",
    "source": "文档中「JVM垃圾回收器」部分",
    "hint": "文档在介绍G1收集器时，强调了它与“前六种收集器”在回收范围上的一个关键不同点。"
  }
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            const wrongCount = answeredCount - correctCount;
            const skippedCount = totalQuestions - answeredCount;
            const accuracy = totalQuestions > 0 ? Math.round((correctCount / totalQuestions) * 100) : 0;
            const errorRate = totalQuestions > 0 ? Math.round((wrongCount / totalQuestions) * 100) : 0;

            const finalScoreHtml =
                '<div class="final-score-container">' +
                '<h2>答题报告</h2>' +
                '<div class="score-summary">' +
                '<div class="score-card">' +
                '<h3>得分</h3>' +
                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
                '</div>' +
                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  